The invention relates to improvements in hydraulic valves, and more particularly to improvements in hydraulic valves of the type wherein an outer or first component (e.g., a substantially cylindrical valve housing or body) has an at least partially cylindrical internal surface adjacent the at least partially cylindrical external surface of a second or inner component, e.g., a piston which is reciprocable in the outer component.
It is customary to provide in one of the internal and external surfaces an outlet which is in communication with a plenum chamber and is or can be sealed from at least one discharge opening in at least one first position of the piston but is free to communicate with such discharge opening in at least one second position of the piston. To this end, the other of the internal and external surfaces is provided with a groove for a sealing element which engages the one surface and is caused to slide across the outlet during movement of the piston between its first and second positions, i.e., the sealing element is located at one side of the outlet in the first position of the piston and at the other side of the outlet in the second position of the piston.
As the sealing element moves across the outlet, it rubs against those portions of the one surface which immediately surround the outlet so that it undergoes extensive wear. In fact, the sealing element can be completely destroyed by tearing or squashing as a result of repeated expansion and contraction as it travels across the outlet. Attempts to avoid rapid and excessive wear upon and extensive damage to or total destruction of the sealing element include the provision of an annular plenum chamber and a set of radial bores which terminate at the one surface and together constitute the outlet of the plenum chamber. The plenum chamber can be provided in the piston and the bores then extend radially outwardly to the external surface of the piston, or the plenum chamber can be provided in the cylinder and the bores then extend radially inwardly to the internal surface. Such solution is often quite unsatisfactory because the combined cross-sectional area of the bores (i.e., the cross-sectional area of the outlet) is too small. Moreover, the cost of making bores in the external surface of the piston or in the internal surface of the housing with a requisite degree of accuracy is high.